This invention relates to a muting circuit used with audio equipment and apparatus which employs analogue signals within the audio frequency range.
In order to intercept signal transmission when handling a non desired signal, a muting circuit is incorporated into the reproducing (playback) circuit or the like of a tape recorder. In this case, the muting circuit functions so as to pass musical and other signals only when the operation mode of a tape desk is playback mode. Unless the muting circuit is provided, clicks may be caused when the operation mode is switched, or some noise attributable to the amplifier system will possibly be reproduced in the stop mode. The muting circuit is expressly vital to a tape deck of a type in which a reproducing head is brought into slidable contact with a recording tape in the fast forward or the rewinding mode.
Conventionally, the muting circuit of the aforesaid type uses a one-stage attenuator combining series resistors with switching elements (bipolar transistor or FET). With such one-stage muting circuit, the attenuation obtained during muting operation may be -40 dB to -50 dB at the most.
In the case of the above-mentioned tape recorder which performs fast forward and rewinding operations with the recording tape slidably touched by the reproducing head, however, the level of the signal to be muted is very high. Therefore, the attenuation ranging from -40 dB to -50 dB would usually be unsatisfactory. It is not advisable to increase the resistance value of the series resistor in order to augment the attenuation degree at muting, because the transmission loss will thence be increased while the muting operation is off. Thus, according to the prior art muting circuit, a high attenuation degree at muting would never be compatible with a small off-muting transmission loss.